With respect to WORM optical recording media that allow recording and reproduction with laser light of a blue wavelength-region (350 nm to 500 nm), the development of blue lasers capable of ultra high density recording has rapidly progressed, and corresponding WORM optical recording media have been developed.
In the conventional WORM optical recording medium, recording pits are formed by emitting laser light onto a recording layer formed of an organic material and thereby causing a change in the refractive index mainly due to the decomposition and modification of the organic material, so that the optical constants and decomposition behavior of the organic material used in the recording layer are important factors in forming satisfactory recording pits.
Accordingly, materials having optical properties and decomposition behavior appropriate for blue wavelength lasers are selected for the recording layers of WORM optical recording media compatible with blue wavelength lasers.
That is, with respect to the conventional WORM optical recording medium having a high-to-low (recording) polarity, recording and reproduction wavelengths are selected so as to be positioned at the longer-wavelength-side edge of a large absorption band in order to increase reflectance in the unrecorded state and to have a large change caused in refractive index by decomposition of the organic material due to exposure to laser light (so as to have a high degree of modulation).
This is because the longer-wavelength-side edge of a large absorption band of an organic material is a wavelength region where absorption coefficients are appropriate and the refractive index is high.
However, there has been found no organic material whose optical properties with respect to blue wavelength laser light have values equivalent to those of a recording material compatible with the conventional red wavelength laser light. This is because the molecular skeleton should be reduced in size or the conjugate system should be shortened in order for an organic material to have an absorption band in the vicinity of blue wavelengths, which, however, results in a decrease in the absorption coefficient, that is, a decrease in the refractive index.
That is, there are many organic materials having an absorption band in the vicinity of blue wavelengths and it is possible to control their absorption coefficients. However, since they do not have high refractive indices, they fail to yield a high degree of modulation.
Further, organic dyes, which are less stable than inorganic materials, have problems in keeping quality and light resistance. Therefore, it has been studied to use an inorganic material for the recording layer of the WORM optical recording medium compatible with blue wavelength laser light.
Examples of the inorganic material recording layer of the WORM optical recording medium compatible with blue wavelength laser light are as follows.
Japanese Laid-Open Patent Application No. 9-286174 (Patent Document 1) proposes a recording layer using the same phase change material as used for a rewritable optical recording medium. However, since the WORM optical recording medium is required to retain information for a long period of time, phase change materials are not sufficient in terms of the storage (retention) characteristic.
Japanese Laid-Open Patent Application No. 2004-79020 (Patent Document 2) proposes stacking inorganic materials in multiple layers and performing recording using their reaction. However, recording using the reaction of multiple layers is not suitable for long-term retention because the reaction progresses with time.
On the other hand, Japanese Laid-Open Patent Application No. 2002-133712 (Patent Document 3) and Japanese Laid-Open Patent Application No. 2003-237242 (Patent Document 4) propose techniques of using metal or metalloid for a recording layer. The former shows a recording layer containing Te, O, and another element, and the latter shows a recording layer containing an imperfect oxide of a transition metal. Further, the latter states that the imperfect oxide of the transition metal may contain an element other than transition metals, but provides only Al as a specific example of such an element. Further, there is no clear definition of transition metal because some definitions include Zn, Y, etc., while others do not, and only W and Mo are described in detail.
No description is given in detail of one object of the present invention, that is, to increase sensitivity, in either the former or the latter document.
A WORM optical recording medium using an oxide for its recording layer is suitable for high density recording because the recording layer is low in thermal conductivity so as to control the thermal interference between recording marks.
In the case of using an oxide for the recording layer, it has been proposed to reduce the degree of oxidation of the oxide (increase the amount of oxygen deficiency) as a method of further improving recording characteristics.
Examples of techniques using a material having less oxygen than its stoichiometric composition in a red or infrared wavelength region include using TeOx (0≦x≦2) (Japanese Laid-Open Patent Application No. S50-46317 [Patent Document 5]), using a material containing at least one selected from the group of TeOx, GeOx, SnOx, BiOx, SbOx, and TlOx and at least one of S and Se (Japanese Patent No. 1444471 [Patent Document 6]), using a lower oxide of Ge (GeOx) containing Te and Sb or a lower oxide of Sb (SbOx) containing Te and Ge (Japanese Patent No. 1849839 [Patent Document 7]), using a low oxide of Ni expressed by NiOx (Japanese Patent No. 2656296 [Patent Document 8]), and an information recording method that forms an image by exposing a low oxide of In to laser light (Japanese Laid-Open Patent Application No. S51-21780 [Patent Document 9]).
Further, Japanese Examined Patent Application Publication No. 7-25209 (Patent Document 10) shows adding an element selected from Sn, In, Bi, Zn, Al, Cu, Ge, and Sb to TeOx with respect to a low oxide in a red wavelength region. In the text of Patent Document 10, BiOx is described as effective when Te, Sb, or Ge is added thereto. However, this technique utilizes a so-called “blackening phenomenon,” where light transmittance changes due to exposure to light and relates to a reversible film that allows a recorded part by blackening to return to its original transmittance through exposure to light.
However, Patent Documents 5 through 10 described above are related to recording media subjected to recording and reproduction in a red or infrared wavelength region, and do not describe techniques supporting blue wavelength laser light.
Therefore, Japanese Laid-Open Patent Application No. 2005-108396 (Patent Document 11) and Japanese Laid-Open Patent Application No. 2005-161831 (Patent Document 12) propose WORM optical recording media that allow high density recording even with blue wavelength laser light, in which a recording layer contains a metal or metalloid oxide, in particular, a bismuth oxide, as a principal component.
These WORM optical recording media, in which the thermal conductivity of the recording layer is low so as to suppress the thermal interference between recording marks, are suitable for high density recording.
In the WORM optical recording medium having a recording layer containing a metal or metalloid oxide as a principal component, the following changes form a principal recording principle, and phase separation due to nucleation and growth is considered to be the basis of recording.                A metal or metalloid oxide is decomposed by recording light or heat due to its emission, so that a metal or metalloid simple substance is generated.        Microcrystallization of the metal or metalloid simple substance occurs.        Microcrystallization of the metal or metalloid oxide occurs.        Phase separation of the metal or metalloid simple substance and the metal or metalloid oxide occurs.        
If multiple metal or metalloid oxides are mixed, phase separation of different oxides occurs.
With respect to recording materials used in WORM optical recording media compatible with blue wavelength laser light, it is easily imaginable that a further increase in recording speed is to be desired, so that it is desirable to further increase recording sensitivity. However, there is a problem in that a metal or metalloid oxide alone is slightly lower in absorption coefficient with respect to recording and reproduction light than, for example, the phase change material as shown in Patent Document 1 or the metal materials as shown in Patent Document 2, so as to have insufficient recording sensitivity compared with higher recording sensitivity to be required in the future.
Some of the inventors of the present invention have found that as a method of further increasing the sensitivity of a recording layer containing a metal or metalloid oxide as a principal component without degrading recording and reproduction characteristics, it is effective to reduce the degree of oxidation of the oxide (increase the amount of oxygen deficiency) (Japanese Laid-Open Patent Application No. 2006-248177 [Patent Document 13]). According to Patent Document 13, a bismuth oxide is caused to contain another oxide, and the oxygen content of the resultant oxide is less than that according to the stoichiometric composition. In this case also, the recording principle is the same as described above. However, the existence of metallic bismuth makes it possible to increase the absorption coefficient of the recording layer with respect to recording light. Accordingly, recording sensitivity is improved. Further, if the oxygen content of the bismuth oxide is less than that according to the stoichiometric composition, crystals of the metallic bismuth are more likely to be deposited so as to yield a higher degree of modulation in the blue wavelength region as well.
Further, it is effective to cause the bismuth oxide to contain another oxide in order to prevent the metal bismuth from aggregating even when the abundance ratio of the metallic bismuth is increased. According to this method, a decrease in the matrix of the bismuth oxide due to the increase in the abundance ratio of the metallic bismuth is compensated for by causing the bismuth oxide to contain another oxide so as to prevent a decrease in the ratio of the matrix to the metallic bismuth, thereby making it possible to prevent aggregation of the metallic bismuth.
By this method, it is possible to cause the oxygen content of the bismuth oxide to be even less than that according to stoichiometric composition compared with the case of forming the recording layer only of the bismuth oxide, which is effective in improving sensitivity.
Further, controlling an increase in the amount of crystalline deposition by adding another oxide to the bismuth oxide results in good formation of small marks, thus making it easy to increase density. Further, the addition of another oxide stabilizes recording marks, thus improving retention (storage) stability.
The recording principle of a WORM optical recording medium having a recording layer of a bismuth oxide containing another oxide is considered to be based on the following changes.
The bismuth oxide is decomposed by recording light or heat due to its emission, so that metallic bismuth is generated.                Microcrystallization of the metallic bismuth occurs.        Microcrystallization of the bismuth oxide occurs.        Microcrystallization of the other oxide occurs.        Phase separation of the metallic bismuth and the bismuth oxide and/or the other oxide occurs.        Phase separation of the bismuth oxide and the other oxide occurs.        
Examples of the above-described method of reducing the degree of oxidation of metal or metalloid include controlling the amount of oxygen or controlling the composition of a sputtering target (adding to the sputtering target a metal or metalloid forming a metal or metalloid oxide that is the principal component of the sputtering target) at the time of forming a film of the metal or metalloid oxide by sputtering.
However, this method has the following problems.                An increase in the amount of oxygen deficiency of the metal or metalloid oxide in the recording layer or the sputtering target degrades recording and reproduction characteristics or recording sensitivity.        An increase in the amount of oxygen deficiency of the metal or metalloid oxide in the sputtering target makes it difficult to prepare the sputtering target or results in poor durability of the sputtering target.        It is difficult to control the uniformity of the degree of oxidation in the directions of film thickness.        
In the case of excessive reduction in the degree of oxidation of the metal or metalloid oxide in the recording layer, if the metal or metalloid has a low melting point, the metal or metalloid is melted by recording, so that phase separation, which is the primary recording principle, is less likely to occur.
Further, the thermal conductivity of the metal or metalloid simple substance is extremely higher than that of the metal or metalloid oxide. Accordingly, if the metal or metalloid simple substance content is higher than or equal to a certain value, this results in degradation of recording sensitivity or degradation of recording and reproduction characteristics such as a reduced degree of modulation.
Further, in the metal or metalloid oxide, as the oxygen content becomes less than that according to the stoichiometric composition (that is, as the abundance ratio of the metal or metalloid increases), larger crystals are deposited so as to make it difficult to record small marks.
Further, in the case where the recording layer is formed only of one type of metal or metalloid oxide, causing the oxygen content in the metal or metalloid oxide to be less than that according to the stoichiometric composition reduces the metal or metalloid oxide that is a matrix. As a result, the metal or metalloid is more likely to aggregate, so as to degrade the uniformity of the degree of oxidation in the directions of film thickness.
Accordingly, in the case of increasing the metal or metalloid content in the metal or metalloid oxide by causing the oxygen content to be less (lower) than that according to the stoichiometric composition, it is desirable to disperse the metal or metalloid in the metal or metalloid oxide using the metal or metalloid oxide as a matrix or to evenly mix the metal or metalloid and the metal or metalloid oxide.
If the metal or metalloid exists non-uniformly with a local concentration, the melting mode becomes a principal recording principle in the location of concentration, which is not preferable. Further, in the location, melting may also be caused by exposure to reproduction light so as to significantly reduce reproduction stability, which is not preferable, either.
That is, it is effective to reduce the degree of oxidation of the metal or metalloid oxide, which is the basis of the recording principle, but excessive reduction in the degree of oxidation causes the loss of a substance that is the basis of the recording principle and also activates a recording principle that hinders the principal recording principle, which may result in degradation of recording and reproduction characteristics or recording sensitivity.
By the way, for example, Japanese Laid-Open Patent Application No. 2006-192885 (Patent Document 14) shows a layer structure where a recording layer containing a metal or metalloid oxide as a principal component and an adjacent layer containing a mixture of a sulfide and an oxide as a principal component are stacked.
Patent Document 14 shows a WORM optical recording medium having a substrate (undercoat layer), a recording layer containing bismuth and/or a bismuth oxide as a principal component, an overcoat layer, and a reflective layer successively stacked from the laser light emission side, wherein the WORM optical recording medium has a reflectance of 35% or less when laser light is emitted onto a flat part of the substrate. In the above-described WORM optical recording medium, the recording layer may have a film thickness of 3 to 20 nm, and the overcoat layer may have a film thickness of 5 to 60 nm or 70 to 150 nm.
As described above, there have been proposed recording layers containing a metal or metalloid oxide as a principal component and configurations where a recording layer containing a metal or metalloid oxide as a principal component and a layer containing at least one compound selected from oxides, nitrides, carbides, fluorides, and sulfides as a principal component are adjacently stacked.
However, there is an increasing demand for higher speed and higher sensitivity.